This invention relates generally to processes for improving the brightness of clays, such as the kaolin clays utilized in the manufacture of paper, and more specifically relates to a process for improving the magnetic benefication of said clays.
Naturally occurring kaolin clay deposits are known to contain discoloring contaminants. Commonly, for example, iron and titanium minerals, as well as other minerals such as montmorillonite and mica, may in various combinations be responsible for such discoloration. The titanium minerals, for example, are commonly present as TiO.sub.2 (e.g. in the form of anatase), and such minerals vary from yellow to dark brown in color. Similarly, in the case of montmorillonite, such mineral is known to be magnetic and to have the ability to capture staining surface iron because of its high cation exchange capacity. These various cited impurities are largely responsible for the yellow-brown shade of many kaolins. Indeed the clay is often rejected as being unsuitable for commercial use, solely on the basis of color, even though its other physical properties, such as the viscosity of clay-water slurries and particle size distribution are within desired limits.
The brightness of clay usually is increased during processing by fractionation: the finer the particles size, the brighter the clay. However, this increase is usually insufficient for the more discolored clays to be rendered acceptable commercially, and additional treatment is required of the fine clay, such as chemical leaching. Leaching with chemicals such as zinc or sodium hydrosulphite thus generally results in improving brightness of the refined clay slurries, although an increase of only 2 to 5 brightness points is usually obtained. There are other known methods of improving brightness of clays, but generally these techniques are quite expensive and do not give sufficient increase in brightness to justify the cost.
In the past, numerous attempts have been made to remove contaminants by the use of magnetic techniques, where the said contaminants are responsive to magnetic attraction. Such techniques utilize the force of a magnetic field gradient to cause differential movement of mineral grains through the said field. In U.S. Pat. No. 3,471,011, for example, magnetic beneficiation of kaolin clay is taught utilizing techniques wherein a slurry of clay in water is subjected to a high intensity magnetic field of at least 8,500 gauss, with the time of retention in the field being from 30 seconds to 8 minutes--in order to separate particles of low magnetic susceptibility from the slurry. More recently in the present assignee's copending application Ser. No. 309,839, filed by Joseph Iannicelli on Nov. 27, 1972, now abandoned as a continuation of Ser. No. 19,169, filed Mar. 13, 1970, now abandoned apparatus and methodology are taught wherein a clay-water slurry is passed through a canister packed with steel wool or similar highly irregular and porous ferromagnetic material. In the presence of an intense magnetic field, the combination set forth has been found to be highly effective in moving the magnetically attractable contaminants, by virtue of the ability of the said system to concentrate the field at myriad points in the canister volume at which attraction may occur.
While magnetic separation techniques such as those set forth above, have therefore been found to be of great value in removing certain contaminants from the clay slurries, and indeed result in several points increase of brightness in the treated products, it is nevertheless found in practice that various of the contaminants are removed only with the greatest difficulty. Among other things, it may be noted that the factor of retention time in the magnetic field has been found to be of great importance in removing certain of these relatively intractable contaminants, with removal being greatly augmented as retention times increase. Unfortunately, however, this factor of retention time is one which must be traded off against the economics of a processing situation. This is to say, that the use of long retention period (and sometimes multiple-pass processing) results in inefficient employment of the large scale and expensive magnetic separating machinery as the slurry passes through the said separator--with consequent losses elsewhere on the production line. Accordingly, it is of considerable interest to discover those techniques which may improve the magnetic separation methodology, both with respect to removal of contaminants previously deemed incapable of treatment; and as well to expediting removal of these contaminants which while known to be magnetically removable have in the past required inacceptable retention periods in the magnetic field to accomplish same.
In accordance with the foregoing, it may be regarded as an object of the present invention, to provide an improved process for magnetically removing discoloring contaminants from clay-water slurries, which process enables highly efficient removal of said contaminants.
It is a further object of the present invention to provide an improved process for magnetically removing discoloring contaminants from clay-water slurries, which enables removal of said contaminants with increased rapidity, and elimination of multiple-pass processing, whereby to render practical high rate and large scale treatment of the said slurries.
It is another object of the present invention, to provide an improved process for magnetically removing discoloring contaminants from clay-water slurries, wherein the efficiency of the process is so improved that it is not required to utilize magnetically saturated matrices.
It is a further object of the present invention, to provide a process for facilitating removal of discoloring contaminants, which process does not require introduction of foreign chemical agents.